The present invention relates to a transmission electron microscope, and more particularly to a transmission electron microscope which has an energy filter capable of obtaining an element distribution image in a small region by forming an image by separating electrons having a specified energy from an electron beam transmitted a sample, and a method of observing element distribution using the transmission electron microscope.
A transmission electron microscope is an apparatus for magnifying and observing an image of a sample using an electron beam and electron lenses, and used for identifying a fine structure of the sample. An energy filter is an apparatus for extracting only electrons having a specified energy by separating the electron beam into a spectrum. By combining the both, electrons having a specified energy can be obtained.
In an electron microscope having an energy filter completed adjustment of the optical axis, an electron microscope image formed only by elastic scattered electrons (zero-loss image) can be obtained by inserting an energy selection slit on the optical axis. When observation is performed by increasing an acceleration voltage of the incident electron beam by .delta.E, electrons lost energy by .delta.E in a sample pass through the energy filter and then pass through the energy selection slit. Therefore, an energy filter image obtained by the electrons lost energy by .delta.E can be obtained by increasing their energy by .delta.E when the zero-loss image is formed.
The electrons transmitted through the sample are lost energy by non-elastic scattering such as plasmon loss core loss, and have energy spectrums. Among the losses, the core loss energy is a value inherent to element composing the sample, and a transmission electron microscope image obtained by electrons affected by a specified energy loss shows two-dimensional distribution corresponding to the element composing the sample. However, energy loss by the non-elastic scattering is broadened over a wide range, and information of the other elements is overlapped on it as a background. A true image cannot be obtained until the background is separated and removed. As the methods of obtaining a specified element distribution image by separating and removing the background, there are proposed the following three kinds of methods.
As shown in FIG. 2, the first method is a method which uses two images, that is, the method uses an energy filter image (post-edge image) B obtained by providing an energy window in a region including core loss energy, and an energy filter image (pre-edge image) A obtained by providing an energy window just before the core loss energy so as to prevent core loss electrons from entering. Initially, these two images A, B are input to a computer using a camera apparatus such as a TV camera. Then, by regarding the pre-edge image A as a background of the post-edge image B, and image subtracting is performed by subtracting the pre-edge image A from the post-edge image B in the computer, and thus a two-dimensional distribution image of a specified element is obtained by separating and removing the background.
As shown in FIG. 3, the second method is a method which uses three images, that is, in addition to the two energy filter images B and C+D used in the first method, the method uses an energy filter image (pre-pre-edge image) A obtained by an energy window in a region not including core loss electrons and an energy region different from in the first method. Similar to the first method, initially, these three images A, B. C+D are input to a computer using a camera apparatus such as a TV camera. Change in a background intensity to energy change is obtained for all pixels from the pre-edge image B and the pre-pre-edge image A using the computer, and an accurate background intensity C of the post-edge image (C+D) is calculated for all pixels over the image. By subtracting the background intensity C obtained in such a manner, a two-dimensional distribution image D of a specified element is obtained by separating and removing the background.
As shown in FIG. 4, the third method is a method which uses two energy filter images as the same as the first method, that is, the method uses a post-edge image B and a pre-edge image A. It can be assumed that in a region where the specified element does not exist, the image intensity decreases at a constant rate as the core loss energy varies. However, in a region where the specified element exists, the ratio of image intensities of the post-edge image and the pre-edge image increases by an amount existing the core loss electrons. Therefore, a two-dimensional distribution image of a specified element is obtained by calculating the ratio of image intensities in each pixel to all pixels over the image and display the calculated ratio of image intensities as a two-dimensional image.
In the first method described above, there is difference between the background intensity used for calculation and the actual background. Therefore, although the calculation process is simple, there is a problem in that the method lacks in quantitative validity.
On the other hand, in the second method, although the actual background can be accurately obtained by using two images, the processing time becomes long because three images are necessary and calculation is performed for all pixels over the images. It is reported that the calculation requires approximately one minutes at minimum (Hiroji Kimoto, Tatsumi Hirano, Katsuhisa Usami, Shigeto Isakozawa, Toshimutsu Taya: The Electron Microscope Society of Japan, the 50.sup.th Scientific Conference Proceeding (1994) 76). Since the series of processes takes a long time, the processed results cannot be fed back during testing.
The third method has an advantage in that it is simple because only two images are required as the same as in the first method, and the processed result cannot be affected by special contrast such as diffract contrast, and artifact due to erroneous processing of background does not exist. However, there is a problem in that it is possible to perform only qualitative evaluation because of lack of quantitative validity.
Although the first method and the third method are short in calculation time compared to the second method, they are difficult to be applied to a sample in which the element distribution is continuously varying with time or a sample which is gradually deforming. When the sample is drifting, calculation such as operation of additionally performing positioning is required. Further, it is disadvantageous from the viewpoint of cost to introduce a high performance computer.
Position and width of the energy window are important factors for image quality of a final image and evaluation of quantitative validity of an element distribution image, and are required to be set to the optimum condition during testing. When real-time processing is impossible, such optimum condition setting cannot help depending on experience of an operator and is technical difficulty. Further, there is a problem in that an error occurs between pixels during calculating background due to noise contained in an image to reduce the S/N ratio.
A method of rapidly displaying an image obtained from the above-mentioned method is proposed in Japanese Patent Application Laid-Open No.8-222169. In this method, a post-edge image and a pre-edge image are alternatively obtained in synchronism with an image recording signal of a camera means such as a TV camera or a SSCCD (slow scan CCD) camera. These images are periodically stored in two frame memory. Image calculation is performed to the two frame memory every pixel, but the calculation result is directly output as a new image signal. The first method or the third method can be realized depending on selecting subtracting process or dividing process in calculation process, and the second method can be realized by multiplying the image intensity by a predetermined constant and performing subtracting process. By these method, a two-dimensional distribution of a specified element can be observed in real-time since the processes can be periodically and continuously performed.
The technology in regard to the element distribution rapid display is one of the methods to solve the aforementioned problems, but it is necessary to provide a means for varying an image intensity at a constant ratio. Further, the image processing is also performed on read-out noise that must have a common average intensity to all images, which becomes a cause to generate artifact. The read-out noise is of a constant noise level not depending on brightness of an image nor exposure time at taking the image. Therefore, there remains noise level which must be eliminated when the inter-image calculation, particularly when inter-image subtraction is performed.